A Review: Synthesis and Applications of Titanium Sub-Oxides

Magnéli phase titanium oxides, also called titanium sub-oxides (TinO2n−1, 4 < n < 9), are a series of electrically conducting ceramic materials. The synthesis and applications of these materials have recently attracted tremendous attention because of their applications in a number of existing and emerging areas. Titanium sub-oxides are generally synthesized through the reduction of titanium dioxide using hydrogen, carbon, metals or metal hydrides as reduction agents. More recently, the synthesis of nanostructured titanium sub-oxides has been making progress through optimizing thermal reduction processes or using new titanium-containing precursors. Titanium sub-oxides have attractive properties such as electrical conductivity, corrosion resistance and optical properties. Titanium sub-oxides have played important roles in a number of areas such as conducting materials, fuel cells and organic degradation. Titanium sub-oxides also show promising applications in batteries, solar energy, coatings and electronic and optoelectronic devices. Titanium sub-oxides are expected to become more important materials in the future. In this review, the recent progress in the synthesis methods and applications of titanium sub-oxides in the existing and emerging areas are reviewed.


Introduction
Titanium sub-oxides, often referred to as Magnéli phase TiO x , comprise a series of different titanium oxides that have the general formula Ti n O 2n−1 (4 ≤ n ≤ 10) [1][2][3][4][5].It is well-known that titanium dioxide (TiO 2 ) is an electrical insulator, as it has a large band gap (anatase: 3.2 eV; 3.0: rutile) [6].However, Magnéli phase titanium oxides are electrically conducting, and the value of electrical resistivity decreases with the increase in the oxygen deficiency [7].Furthermore, Magnéli phase TiO x are found to be more stable than carbon in electrochemically oxidizing conditions [1,8].These titanium sub-oxides have attracted much recent attention as promising new conducting materials because they are electrically conducting, and are highly stable towards chemical corrosion.
The earliest phase analysis using X-ray methods on the oxygen-titanium system was carried out by Ehrlich, who reported the existence of three intermediary titanium oxides [9,10].In the 1950s, a phase diagram of a titanium-oxygen system was constructed from data in the literature by DeVries et al. [11]; later, a comprehensive phase analysis of TiO x was studied by the group of Arne Magnéli, and a number of phases in the titaniumoxygen system were reported, including Ti 4 O 7 , Ti 5 O 9 , Ti 6 O 11 , Ti 7 O 13 , Ti 8 O 15 , Ti 9 O 17 and Ti 10 O 19 [12].The electrical properties of these titanium oxides were studied by Bartholomew et al., and it was found that titanium sub-oxides have semiconductor-to-metal transitions at certain temperatures and their electrical conductivities change with the oxygen content of those materials [13].Ti 4 O 7 has the highest electrical conductivity among Magnéli phase TiO x at room temperature [1].Magnéli phase TiO x are generally synthesized by the reduction of TiO 2 .The reduction sequence is as follows: TiO 2 → Ti n O 2n−1 (n > 10) → Ti n O 2n−1 (4 < n < 10) → Ti 3 O 5 → Ti 2 O 3 → TiO → Ti 2 O [14].Oxygen defect formations and its concentration depend on the synthesis conditions.Ti 4 O 7 can be expressed as TiO 1.75 , as the ratio of O/Ti of Ti 4 O 7 is 1.75.The Magnéli phase Ti n O 2n−1 are the intermediate products.It is critical to have wellcontrolled preparation conditions to synthesize each Magnéli phase, with high chemical and phase purities that have significant effects on the intrinsic properties such as electrical, optical behavior and their catalytic activity.
It is increasingly important to synthesize nanostructured titanium sub-oxides because of their particular properties resulting from the high surface areas of the materials [15][16][17][18].Progress has been made in applying nanostructured titanium sub-oxides in the areas of fuel cells, water treatment, batteries and so on.To update the recent research progress of synthesis and applications of titanium sub-oxides, this review will discuss and highlight the recent developments in the synthesis and applications of Ti 4 O 7 in the fields of energy, environment, catalysts and others, as well as future directions for research.

Synthesis Methods
The phase diagram of the Ti-O system (Figure 1) shows various stable phases at different O/Ti ratios.The region at the right of the diagram contains the discrete Magnéli phases of Ti n O 2n−1 (n = 4-10) and TiO 2 .At sufficiently elevated temperatures, TiO 2 can be reduced to a lower oxidation state such as Magnéli series, including Ti 4 O 7 .To obtain individual phases such as Ti 4 O 7 in the Magnéli series, the condition for reduction of TiO 2 needs to be carefully controlled.The key parameters for the synthesis of Ti 4 O 7 , as well as for other Magnéli phases, include temperature, time, reducing atmosphere and reducing agents.Hydrogen, carbon, metal and hydride can be used as reducing agents.
sequence is as follows: TiO2 → TinO2n−1 (n > 10) → TinO2n−1 (4 < n < 10) → Ti3 TiO → Ti2O [14].Oxygen defect formations and its concentration depend on conditions.Ti4O7 can be expressed as TiO1.75, as the ratio of O/Ti of Ti4O7 is 1 néli phase TinO2n−1 are the intermediate products.It is critical to have well-con aration conditions to synthesize each Magnéli phase, with high chemical an ties that have significant effects on the intrinsic properties such as electric havior and their catalytic activity.
It is increasingly important to synthesize nanostructured titanium su cause of their particular properties resulting from the high surface areas of [15][16][17][18].Progress has been made in applying nanostructured titanium sub areas of fuel cells, water treatment, batteries and so on.To update the recent gress of synthesis and applications of titanium sub-oxides, this review wil highlight the recent developments in the synthesis and applications of Ti4O of energy, environment, catalysts and others, as well as future directions for

Synthesis Methods
The phase diagram of the Ti-O system (Figure 1) shows various stable ferent O/Ti ratios.The region at the right of the diagram contains the disc phases of TinO2n−1 (n = 4-10) and TiO2.At sufficiently elevated temperature reduced to a lower oxidation state such as Magnéli series, including Ti4O7.T vidual phases such as Ti4O7 in the Magnéli series, the condition for reduction to be carefully controlled.The key parameters for the synthesis of Ti4O7, other Magnéli phases, include temperature, time, reducing atmosphere agents.Hydrogen, carbon, metal and hydride can be used as reducing agen

Reduction of TiO2 by Hydrogen
At sufficiently elevated temperatures, hydrogen (H2) or a mixture hydro such as argon (Ar) is used to reduce TiO2 into the titanium sub-oxides [1].T process can be considered to be a reaction of oxygen being removed progr TiO2.The reaction for the synthesis of Ti4O7 through hydrogen reduction Equation (1): 4TiO2 + H2 = Ti4O7 + H2O

Reduction of TiO 2 by Hydrogen
At sufficiently elevated temperatures, hydrogen (H 2 ) or a mixture hydrogen-inert gas such as argon (Ar) is used to reduce TiO 2 into the titanium sub-oxides [1].The reduction process can be considered to be a reaction of oxygen being removed progressively from TiO 2 .The reaction for the synthesis of Ti 4 O 7 through hydrogen reduction is shown in Equation ( 1): The reduction reaction to produce Ti 4 O 7 is carried out at sufficiently elevated temperature, generally higher than 1000 The reduction reaction of TiO 2 is carried out in a flow of hydrogen in a reactor that is heated externally to maintain a high temperature.As Ti 4 O 7 is the last in the reduction sequence, the synthesis of Ti 4 O 7 needs higher temperature, longer reduction time or a combination of two, compared with the parameters to the formation of other Magnéli series.The reaction temperature, reaction time, gas composition and size of TiO 2 particles are important factors for the synthesis of Magnéli phases [2,3].A summary of the synthesis of Ti 4 O 7 through hydrogen reduction can be found in Table 1.

Reduction by Carbon
Titanium dioxide can be reduced by carbon in an inert atmosphere to produce various titanium sub-oxides, as shown in Equation (2): The carbothermal reduction of TiO 2 is a complex process in which the oxygen in TiO 2 is progressively removed by carbon.TiO 2 is initially reduced to Ti n O 2n−1 , Ti 3 O 5 , Ti 2 O 3 and TiC x O y [25,[37][38][39], but an over stoichiometric carbon/TiO 2 ratio may lead to the formation of TiC x O y , not titanium sub-oxides.Ti n O 2n−1 phases are only formed as intermediates [25,37].To prepare titanium sub-oxides through the carbothermal reduction of TiO 2 , the stoichiometric carbon/TiO 2 ratio is important for the control of the phases formed.The carbothermal reduction of TiO 2 can be carried out in different gas atmospheres or in a vacuum.Li et al. synthesized Ti 4 O 7 by reacting TiO 2 anatase (100 nm) with carbon black at 1020 • C for 0.5-2 h in argon and in a vacuum [26].The study indicated that at the same temperature, the extent of carbothermal reduction of titanium dioxide is dependent on the molar ratio of TiO 2 /C, and excessive carbon may lead to over reduction down the sequence of titanium sub-oxides.Ti 4 O 7 with a purity of 98.5% was obtained in argon at 1100 • C. Dewan et al. studied the carbothermal reduction of TiO 2 in hydrogen, helium and argon through temperature-programmed reduction experiments [40].In argon and helium, the carbothermal reduction of TiO 2 started at 850 • C. In hydrogen, they found that the phases in a sample after being reduced to 915 Titanium sub-oxide fibers with high electrical conductivity have been prepared by reducing TiO 2 in a carbon black micro-environment [29].Organic polymers or compounds can be used to synthesize titanium sub-oxides.The carbon in the organic polymers is used as a carbon source for reducing TiO 2 or other titanium-containing compounds.These organic polymers or compounds include poly (ethyleneimine), polyethyleneglycol [41], poly (styrene-b-2-vinylpyridine) [42], resol [43], glucose [44] and poly (vinyl alcohol) [27].A summary of the various preparation methods for titanium sub-oxides using the carbon reduction method can be found in Table 1.

Reduction by Metals
Metals can be used to reduce TiO 2 to form titanium sub-oxides [31,32].Calcium, aluminum, sodium, silicon and titanium have been used to reduce TiO 2 .For example, by controlling the ratio of metallic titanium and TiO 2 , metallic titanium (Ti) can be used to reduce TiO 2 to obtain various titanium sub-oxides through a reaction shown in Equation (3).
Andersson et al. synthesized various titanium sub-oxides by the reduction of TiO 2 with titanium metal under argon, and established the different phases from X-ray diffraction determinations [9].Strobel et al. used Ti and TiO 2 to react in situ in carefully out-gassed transport tubes.Cl 2 and tellurium tetrachloride were used as transporting agents to synthesize crystals of Ti n O 2n−1 with n = 2 to 9 [45].Gusev et al. developed a method for the synthesis of titanium sub-oxides by reducing TiO 2 with titanium.This method involved the mechanical activation and annealing in argon at temperatures of 1333-1353 K for 4 h [46].It is worth noting that the synthesis of titanium sub-oxides using TiO 2 and Ti can be viewed to be an oxidation reaction in which Ti is oxidized by TiO 2 .Theoretically, oxidation of Ti is one of the possible ways to obtain titanium suboxides.However, oxygen is highly reactive and can oxidize Ti directly to TiO 2 easily.Fine Ti powder is far more difficult to prepare than TiO 2 .A summary of the synthesis of titanium sub-oxides by metal reduction can be found in Table 1.

Reduction by Hydride
Metal hydrides have strong reducing reactivity, even at low temperatures.The reduction of TiO 2 to titanium sub-oxides could occur at low temperatures to avoid significant sintering and crystal growth of particles in the formation process of titanium sub-oxides.Therefore, metal hydrides could be used to synthesize nanostructured titanium sub-oxides using nanostructured TiO 2 as a starting material.Nagao et al. synthesized titanium suboxides by reacting TiO 2 with TiH 2 at 550 • C [34].The nanoparticles of a series of phases of titanium sub-oxide including Ti 2 O 3 , Ti 3 O 5 , Ti 4 O 7 and Ti 8 O 15 were obtained by changing the molar ratios of TiO 2 /TiH 2 .Other hydride reduction methods for titanium sub-oxide synthesis can be found in Table 1.

Synthesis of Nanostructured Titanium Sub-Oxides
It has become increasingly important to synthesize nanostructured titanium suboxides because of their particular properties resulting from the high surface areas of the materials.Nanostructured non-stoichiometric TiO 2−x titanium sub-oxides, titanium suboxides Ti n O 2n−1 in particular, have emerged as alternatives to TiO 2 in applications of clean energy generation, and as catalysts for degrading harmful compounds and others [47][48][49][50][51][52].Although titanium sub-oxides can be synthesized by the reduction of TiO 2 using hydrogen or carbon, the sizes of synthesized titanium sub-oxide particles are usually in the order of micrometers, because these reduction reactions occur at high temperatures (generally over 1000 • C) and proceed for hours.Under these conditions, TiO 2 and formed titanium sub-oxide particles undergo sintering and crystal growth, leading to the formation of much larger particles.To synthesize nanostructured titanium sub-oxides, more reactive titaniumcontaining starting materials, stronger reducing agents or alternative reaction techniques are required for the reduction reactions to be carried out under milder reaction conditions such as lower temperatures or short reaction times.He et al. fabricated Ti8O15 nanowires using an evaporation-dep method [15].The synthesized Ti8O15 nanowires were ∼30 nm in diam Figure 3), and were found to have an electrical conductivity of 20.6 S cm   He et al. fabricated Ti8O15 nanowires using an evaporation method [15].The synthesized Ti8O15 nanowires were ∼30 nm in d Figure 3), and were found to have an electrical conductivity of 20.6  Portehault et al. developed a new bottom-up approach to synthesize various nanoscaled Magnéli phases under mild conditions [41].In this method, titanium (IV) ethoxide was reacted with amino-or ethoxy-containing oligomers or polymers.The resulting clear gels were heated at different temperatures under N 2 or Ar.Ti n O 2n−1 compounds (n = 3, 4, 5, 6, 8) were obtained for the first time as nano-Magnéli phases with specific surface areas from 55 to 300 m 2 g −1 .The synthesis steps for the Magnéli/carbon nanocomposites are illustrated in Figure 4.  Huang et al. synthesized nanocrystalline Ti2O3, Ti3O5 and Ti4O7 using a synthesis method that combines sol-gel and vacuum-carbothermic processes [44].Yao et al. successfully synthesized Ti4O7 using TiO(NO3)2 as a starting material in a hydrogen atmosphere at 1000 °C for 6 h [22].The SEM images clearly showed that the synthesized titanium suboxides are spherical particles with an average particle size of approximately 250 nm.Davydov synthesized Ti4O7 nanopowder with an average size of 115 ± 30 nm using a twostep procedure.In the first step, titanium (III) oxalate particles with controlled sizes were produced by reacting metallic Ti with oxalic acid in a heated aqueous solution.In the second step, Ti4O7 was prepared through high-temperature calcination of titanium (III) oxalate particles in a flowing hydrogen gas [62].This synthesis process is similar to the process that has been used to prepare titanium oxycarbide nanoparticles [63].Tominaka et al. synthesized Ti2O3 nanoparticles by heating TiO2 nanoparticles (10-30 nm) and CaH2 powder at 350 °C [35].
Ioroi et al. synthesized nanoparticles of titanium sub-oxides by irradiating TiO2 particles dispersed in liquid with a pulsed UV laser [64].Xu et al. developed a synthesis process to prepare titanium sub-oxide nanoparticles via a thermal plasma method, using metatitanic acid H2TiO3 (TiO(OH)2) as a starting material.The prepared titanium sub-oxides nanoparticles are spherical, with particle sizes in the range of 20-100 nm [23].Fukushima et al. synthesized Ti4O7 nanoparticles with different sizes by carbothermal reduction using a multimode microwave apparatus [28].Takeuchi et al. synthesized 60 nm Ti4O7 nanoparticles via carbothermal reduction of TiO2 nanoparticles using polyvinylpyrrolidone as the carbon source.The carbothermal reduction was carried out using 2.45 GHz microwave irradiation at 950 °C for 30 min.The results of this study demonstrate that microwave heating can drastically reduce the heating time to avoid excessive sintering and crystal growth of Ti4O7 in a conventional carbothermal reduction process [65].Arif et al. prepared chain-structured titanium sub-oxides with diameters under 30 nm using a thermal-induced plasma process.The synthesized titanium sub-oxide nanoparticles consisted of a mixture of several Magnéli phases.After a heat treatment, as-synthesized titanium suboxides nanoparticles were found to have low electrical resistivity [66].
A summary of synthetic methods for nanostructured titanium sub-oxides is reported in Table 2.A comparison among the synthesis methods to highlight the advantages, limitations and characteristics of the prepared sub-oxides is presented in Table 3. Huang et al. synthesized nanocrystalline Ti 2 O 3 , Ti 3 O 5 and Ti 4 O 7 using a synthesis method that combines sol-gel and vacuum-carbothermic processes [44].Yao et al. successfully synthesized Ti 4 O 7 using TiO(NO 3 ) 2 as a starting material in a hydrogen atmosphere at 1000 • C for 6 h [22].The SEM images clearly showed that the synthesized titanium sub-oxides are spherical particles with an average particle size of approximately 250 nm.Davydov synthesized Ti 4 O 7 nanopowder with an average size of 115 ± 30 nm using a two-step procedure.In the first step, titanium (III) oxalate particles with controlled sizes were produced by reacting metallic Ti with oxalic acid in a heated aqueous solution.In the second step, Ti 4 O 7 was prepared through high-temperature calcination of titanium (III) oxalate particles in a flowing hydrogen gas [62].This synthesis process is similar to the process that has been used to prepare titanium oxycarbide nanoparticles [63] chain-structured titanium sub-oxides with diameters under 30 nm using a thermal-induced plasma process.The synthesized titanium sub-oxide nanoparticles consisted of a mixture of several Magnéli phases.After a heat treatment, as-synthesized titanium sub-oxides nanoparticles were found to have low electrical resistivity [66].
A summary of synthetic methods for nanostructured titanium sub-oxides is reported in Table 2.A comparison among the synthesis methods to highlight the advantages, limitations and characteristics of the prepared sub-oxides is presented in Table 3. Carbothermal reduction of cross-linked titanium ethoxide with polyethylene glycol at ~950 • C in Ar stream [58] Magnéli phases with specific surface areas from 55 to 300 m 2 g −1 The gels made from titanium (IV) ethoxide and amino-or ethoxy-containing oligomers or polymers were heated at different temperatures under N 2 or Ar [41] Nanocrystalline Ti 2 O 3 , Ti

Applications of Titanium Sub-Oxides
The structures of Magnéli phase titanium oxides are based on the rutile TiO 2 crystal lattice.Rutile TiO 2 is made up of octahedra having a titanium atom in the center and oxygen atoms at each corner.Shared edge or corner oxygen atoms link adjacent octahedra, as shown in Figure 5.The crystal structure of titanium sub-oxides can be described as a structure having a two-dimensional chain of titanium dioxide in which titanium atoms locate at the center and oxygen atoms locate at the corners in an octahedral structure [67,68].In Ti n O 2n−1 , every nth layer has an oxygen deficiency, which leads to shear planes in the crystal structure.The Ti 4 O 7 crystal has three octahedral TiO 2 layers and one TiO layer.As a result of the vacancy of oxygen atoms, the TiO layer causes titanium atoms to be closer together.
shown in Figure 5.The crystal structure of titanium sub-oxides can be described as a structure having a two-dimensional chain of titanium dioxide in which titanium atoms locate at the center and oxygen atoms locate at the corners in an octahedral structure [67,68].In TinO2n−1, every nth layer has an oxygen deficiency, which leads to shear planes in the crystal structure.The Ti4O7 crystal has three octahedral TiO2 layers and one TiO layer.As a result of the vacancy of oxygen atoms, the TiO layer causes titanium atoms to be closer together.The unique crystal structure makes titanium sub-oxide materials have attractive properties, such as high conductivity, superior chemical stability and electrochemical stability [69].As shown in Table 4, the conductivity of Ti4O7 material is the highest among the Magnéli phase materials.Research shows that Ti4O7 is highly stable in acidic or alkali conditions.Some studies indicated that the expected half-life of Ti4O7 is 50 years in 1.0 M H2SO4 at room temperature [70].As a result of their remarkable electrical conductivity, electrochemical stability, costeffectiveness and environmentally friendly natures, titanium sub-oxides are also considered to have potential as a superior anode material for wider electrochemical applications [67,68].Research has shown that Magnéli phases have a catalytic property.Among the Magnéli phases, Ti4O7 exhibits the greatest catalytic property [45,69].It has a wide electrochemical window with regard to water oxidation and reduction [70][71][72]; thus, it can be used for electrochemical treatment of pollutants in water.Titanium sub-oxides are generally prepared in the form of powders.More recently, two-dimensional films and threedimensional porous materials of titanium sub-oxides have been successfully fabricated.Advances in the development of multiple dimensional titanium sub-oxide materials has led to new applications.The unique crystal structure makes titanium sub-oxide materials have attractive properties, such as high conductivity, superior chemical stability and electrochemical stability [69].As shown in Table 4, the conductivity of Ti 4 O 7 material is the highest among the Magnéli phase materials.Research shows that Ti 4 O 7 is highly stable in acidic or alkali conditions.Some studies indicated that the expected half-life of Ti 4 O 7 is 50 years in 1.0 M H 2 SO 4 at room temperature [70].As a result of their remarkable electrical conductivity, electrochemical stability, costeffectiveness and environmentally friendly natures, titanium sub-oxides are also considered to have potential as a superior anode material for wider electrochemical applications [67,68].Research has shown that Magnéli phases have a catalytic property.Among the Magnéli phases, Ti 4 O 7 exhibits the greatest catalytic property [45,69].It has a wide electrochemical window with regard to water oxidation and reduction [70][71][72]; thus, it can be used for electrochemical treatment of pollutants in water.Titanium sub-oxides are generally prepared in the form of powders.More recently, two-dimensional films and three-dimensional porous materials of titanium sub-oxides have been successfully fabricated.Advances in the development of multiple dimensional titanium sub-oxide materials has led to new applications.

Catalysis Support in Fuel Cells
Proton exchange membrane fuel cells (PEMFCs) are a clean energy technology that has made significant advances in recent decades [73,74].However, the high cost of the component materials and the low stability of the electrodes are major barriers for their large-scale commercial applications in some areas [75].PEMFCs use Pt catalysts in the form of nanoparticles dispersed on a support material.The nature of the support materials can have a significant influence on the electro-activity and durability of the Pt catalysts [76][77][78].Carbon materials are the most common support material for PEMFCs.However, carbonsupported Pt catalysts are prone to corrosion under the harsh operating conditions [79,80], which can severely affect the performance of PEMFCs and reduce the operational lifetime of the fuel cell electrodes.Titanium sub-oxides are considered to be promising support materials for PEMFCs due to the high thermal and oxidative stability, electronic conductivity and strong interactions between Pt nanoparticles and titanium sub-oxide support

Electrocatalytic Degradation for Wastewater Treatment
Titanium sub-oxides have been characterized as an ideal choice of anode for the electrochemical treatment of many pollutants.Chen et al. decomposed trichloroethylene (TCE) and chloroform (CF) in an electrochemical cell using a titanium sub-oxide ceramic sheet plated with Pt or Pd as the working electrode.The decomposition kinetics was found to be of the first order for TCE and CF [87].Kearney et al. used Ebonex (a titanium sub-oxide ceramic) electrodes for treating nitrate-contaminated water.Complete de-nitrification was achieved using an Ebonex cathode and a stable anode based on Ti/IrO 2 or Ti/RuO 2 [88].Yang et al. examined the degradation of perfluorooctanesulfonate in electrochemical oxidation processes, using an anode made from Ti 4 O 7 .The decomposition rate of perfluorooctanesulfonate was shown to be pseudo-first-order.This study illustrates the promise of Ti 4 O 7 electrodes for degrading per-and polyfluoroalkyl compounds and co-contaminants in groundwater [89].Ganiyu et al. reported a study of the electrochemical degradation of the antibiotic amoxicillin in aqueous solution.The Ti 4 O 7 anode of the cell was prepared using plasma spraying technology.The oxidative degradation of amoxicillin by hydroxyl radicals was assessed as a function of the applied current, and was found to follow pseudo-first-order kinetics.Comparative studies of mineralization efficiency showed that a Ti 4 O 7 anode performed better for the removal of total organic carbon (TOC) than the classical dimensional stable anode and Pt anode.Ti 4 O 7 anodes could provide a costeffective alternative to boron doped diamond anodes in electro-oxidation processes [90].Teng et al. investigated the electrochemical oxidation of sulfadiazine using a Ti/Ti 4 O 7 mesh anode.Their results showed that electrochemical oxidation could achieve almost 100% removal of sulfadiazine in 60 min under the conditions of 0.05 mol L −1 Na 2 SO 4 , pH = 6.33 and current density of 10 mA cm −2 .It was found that Ti/Ti 4 O 7 mesh anodes were very stable in the treatment of actual pharmaceutical wastewater, and had a large electrochemically active surface area due to the network structure of the Ti/Ti 4 O 7 mesh anode [91].Further research will continue to improve the performance of titanium suboxide electrodes through optimizing the fabrication process of the electrodes and further integrating them with other technologies for more efficient applications.

Reactive Electrochemical Membrane
One recent research advancement in water treatment concerns the development of technologies that incorporate multiple treatment methods into a single technology to increase the efficiency and reduce the complexity of water treatment.A novel technology known as reactive electrochemical membranes (REM) combines membrane filtration with electrochemically advanced oxidation processes.In this REM technology, titanium suboxide materials serve as both a ceramic membrane for filtration and a reactive electrode surface for oxidizing contaminants [92].Zaky et al. used Ti 4 O 7 REM to investigate the removal of p-substituted phenolic compounds in water.They demonstrated that the REM was active for both direct anodic oxidation and production of OH• radicals to degrade phenolic compounds [93].Guo et al. synthesized a novel REMs for water treatment using tubular asymmetric TiO 2 ultrafiltration membranes as precursors.REMs composed of high purity Ti 4 O 7 showed optimal reactivity.The performance of REMs was assessed by measuring the outer-sphere charge transfer (Fe(CN)6 4− ) and oxidation of organic compounds through both direct oxidation and generation of OH•.In an optimal condition, the removal rate for oxalic acid was determined to be 401.microfiltration membrane as the filter and the anode.The REM system was evaluated for the performance in deactivating Escherichia coli (E.coli) in water at various current densities.The results showed that the concentration of E. coli was reduced from 6.46 log CFU/mL to 0.18 log CFU/mL, after passing through the Ti 4 O 7 microfiltration membrane filter.The scanning electron microscope and extracellular protein analysis showed that the membrane filtration effect and direct oxidation generated from the REM system are responsible for the observed bacteria removal and inactivation [98].Research is continuing to optimize the electrode fabrication process, and to develop titanium sub-oxide electrodes doped with active electrode materials to further increase the efficiency of water treatment processes, prolonging electrode working life and expanding the degradation of complex pollutants.

Batteries
The lithium-sulfur battery (LSB) is considered to be one of the next-generation technologies for future batteries because of its remarkable specific capacity of 1675 mA h g −1 and the availability of low-cost sulfur [60,99].However, the development of commercial LSBs needs to resolve the issues of low sulfur utilization and poor cyclability, which are caused by a number of factors, such as the low conductivity of sulfur, the high solubility of the lithium polysulfides, passivation of the reactive surface of lithium anodes, and so on.To address these issues, one of the research efforts is to develop host materials to limit the movement of the lithium polysulfides in the sulfur cathode.Tao et al. discovered that conductive Ti 4 O 7 was a highly effective matrix to bind with sulfur species.Ti 4 O 7 -S cathodes exhibit higher reversible capacity and improve cycling performance over previously developed TiO 2 -S cathodes.The strong adsorption of sulfur species on the low-coordinated Ti sites of Ti 4 O 7 was attributed to the improved performance of Ti 4 O 7 -S cathodes [100].Wei et al. prepared mesoporous Ti 4 O 7 microspheres that exhibit interconnected mesopores (20.4 nm), large pore volume (0.39 cm 3 g −1 ), and a high surface area (197.2 m 2 g −1 ).The sulfur cathode embedded with a matrix of mesoporous Ti 4 O 7 microspheres exhibits a superior reversible capacity and a low decay in capacity.The improved electrochemical performance is due to the strong chemical bonding of the lithium polysulfides to Ti 4 O 7 , and trapping in the mesopores and voids of the matrix [43].Zhang et al. reported a facile approach to prepare nanostructured Ti 4 O 7 with different morphologies.Ti 4 O 7 nanorods and nanoparticles were prepared.The as-prepared Ti 4 O 7 nanorods and nanoparticles were examined as a sulfur host for Li-S batteries.The electrochemical tests showed that the Ti 4 O 7 nanorods exhibited better performance in cycle stability and rate capacity compared with Ti 4 O 7 nanoparticles.This confirmed that the morphology of Ti 4 O 7 could influence its electrochemical performance for lithium sulfur batteries [101].Wu et al. synthesized a composite containing carbon nanotubes and nanosized Ti 4 O 7 (oCNTs-Ti 4 O 7 ), and coated the composite on the surface of the separator.Compared with a common separator, the separator modified with the oCNTs-Ti 4 O 7 layer exhibited significantly improvement in the utilization of active substances, and restrained the shuttling effect of polysulfides.The Li-S battery fabricated using the separator modified with the oCNTs-Ti 4 O 7 layer showed great enhancements in cycle and rate performance, as well as other in electrochemical properties [102].Yu et al. fabricated a lithium-sulfur battery cathode containing 7.5 wt% to 10 wt% Ti 4 O 7 .The addition of Ti 4 O 7 as a conductive additive into the cathode resulted in better rate capability and reversible cycling performance.The high electronic conductivity and surface adsorption of the polysulfides of Ti 4 O 7 were attributed to the improvement in the electrochemical performance.This research also showed an effective way to improve the performance of lithium-sulfur batteries [103].Titanium sub-oxides have also been used to improve the performance of other types of batteries such as lead-acid batteries and Zn-air batteries [104,105].

Summary and Outlook
In this review, recent progress in the synthesis and applications of Magnéli phase titanium oxides was reviewed.Titanium sub-oxides are synthesized through the reduction of titanium dioxide (TiO 2 ) using hydrogen, carbon, metals or metal hydrides as reduction agents.The particle sizes of as-synthesized titanium sub-oxides are generally in the micrometer range, based on the conventional synthesis methods.However, progress has been made to synthesize nanostructured titanium sub-oxides through optimizing thermal reduction processes, using more powerful reduction agents or using new titanium-containing precursors [15,23,28,62,65,66].Magnéli phase titanium oxides have numerous applications in electrodes, fuel cells, degradation of pollutants, batteries and coatings.Among these compounds, Ti 4 O 7 has received the most widespread attention due to its excellent electrical conductivity, and chemical and electrochemical stability.More recently, Magnéli phase titanium oxides as functional materials or additives have been used to enhance the performance of electro-catalysts, cathodes in batteries, advanced electrochemical oxidation processes, solar cells, electronic materials, sensors and coatings [95,110,111,[118][119][120][121][122].It is expected that further research will be continue to optimize synthesis processes of Magnéli phase titanium oxides to further increase the electrochemical and catalytic properties, and to improve the performance of devices containing Magnéli phase titanium oxides through optimizing the fabrication process and further integrating with other technologies for more efficient applications.Titanium sub-oxides are expected to become more important materials for sustainability in the future.
Han et al. prepared Ti 8 O 15 nanowires and Ti 4 O 7 fibers by heating H 2 Ti 3 O 7 nanowires in hydrogen at 850 • C and 1050 • C[53].Hydrogen trititanate H 2 Ti 3 O 7 is one of the compounds in the series of titanates (M 2 Ti n O 2n+1 , M = H, Na, or K).The synthesis process has two steps.Firstly, H 2 Ti 3 O 7 nanowires are prepared by reacting TiO 2 particles with NaOH in an autoclave at 150-180 • C for 2-5 days, and then purified using the acid washing method[54][55][56].Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed that prepared H 2 Ti 3 O 7 were nanowires of 30-200 nm in diameter and up to 10 µm long.Secondly, prepared H 2 Ti 3 O 7 nanowires were reduced in hydrogen for 1-4 h at 800-1050 • C. In the hydrogen reduction reaction at 850 • C, H 2 Ti 3 O 7 nanowires changed into Ti 8 O 15 nanorods or nanoparticles, as shown in Figure2.By heating H 2 Ti 3 O 7 in hydrogen at 1050 • C, the product formed was Ti 4 O 7 .The TEM image shows that most products are in the form of fibers, with diameters of approximately 1 µm.He et al. fabricated Ti 8 O 15 nanowires using an evaporation-deposition synthesis method[15].The synthesized Ti 8 O 15 nanowires were ∼30 nm in diameter (as shown in Figure3), and were found to have an electrical conductivity of 20.6 S cm −1 .Zhang et al. prepared pure Ti 4 O 7 particles with diameters of 200-500 nm in hydrogen at 850 • C using peroxotitanium acid H 4 TiO 5 (Ti(OH) 3 O-O-H) as a starting material [57].H 4 TiO 5 was prepared by treating titanium powder with NH 3 •H 2 O and H 2 O 2 .Pang et al. synthesized Ti 4 O 7 , using a simple polymer-mediated route in which the cross-linked titanium ethoxide with polyethylene glycol was treated by carbothermal reduction at ~950 • C in an Ar stream.TEM images revealed that the material primarily comprises ~8-20 nm Ti 4 O 7 crystals.The sulfur composites Ti 4 O 7 /S-60 or Ti 4 O 7 /S-70 were prepared with either 60 or 70 wt% sulfur using a melt-diffusion method at 155 • C [58].Ti 4 O 7 was used to prepare Ti 4 O 7 /S cathodes for lithium-sulfur cells [59-61].

Figure 2 .
Figure 2. Ti8O15 nanorods prepared by reducing H2Ti3O7 at 850 °C in hydrogen the product; (b) SEM image of the product; (c) low magnification TEM image (d) a high-magnification TEM image of part of a nanorod (Reprinted from ref. [ American Institute of Physics).

Figure 2 .
Figure 2. Ti 8 O 15 nanorods prepared by reducing H 2 Ti 3 O 7 at 850 • C in hydrogen; (a) XRD pattern of the product; (b) SEM image of the product; (c) low magnification TEM image of the product; and (d) a high-magnification TEM image of part of a nanorod (Reprinted from ref. [53], copyright 2008, American Institute of Physics).

Figure 2 .
Figure 2. Ti8O15 nanorods prepared by reducing H2Ti3O7 at 850 °C in hydr the product; (b) SEM image of the product; (c) low magnification TEM im (d) a high-magnification TEM image of part of a nanorod (Reprinted from American Institute of Physics).

Figure 3 .
Figure 3. (a,b) SEM images of the Ti8O15 nanowires; (c) SEM image of the c nanowires; (d,e) TEM images of the Ti8O15 nanowires; (f) HRTEM image printed from ref. [15], copyright 2015, The Royal Society of Chemistry).

Figure 3 .
Figure 3. (A,B) SEM images of the Ti 8 O 15 nanowires; (C) SEM image of the cross-section of the Ti 8 O 15 nanowires; (D,E) TEM images of the Ti 8 O 15 nanowires; (F) HRTEM image of Ti 8 O 15 nanowires (Reprinted from ref. [15], copyright 2015, The Royal Society of Chemistry).

Materials 2023 ,
16,  x FOR PEER REVIEW 7 of 18 was reacted with amino-or ethoxy-containing oligomers or polymers.The resulting clear gels were heated at different temperatures under N2 or Ar.TinO2n−1 compounds (n = 3, 4, 5, 6, 8) were obtained for the first time as nano-Magnéli phases with specific surface areas from 55 to 300 m 2 g −1 .The synthesis steps for the Magnéli/carbon nanocomposites are illustrated in Figure4.
. Tominaka et al. synthesized Ti 2 O 3 nanoparticles by heating TiO 2 nanoparticles (10-30 nm) and CaH 2 powder at 350 • C [35].Ioroi et al. synthesized nanoparticles of titanium sub-oxides by irradiating TiO 2 particles dispersed in liquid with a pulsed UV laser [64].Xu et al. developed a synthesis process to prepare titanium sub-oxide nanoparticles via a thermal plasma method, using metatitanic acid H 2 TiO 3 (TiO(OH) 2 ) as a starting material.The prepared titanium sub-oxides nanoparticles are spherical, with particle sizes in the range of 20-100 nm [23].Fukushima et al. synthesized Ti 4 O 7 nanoparticles with different sizes by carbothermal reduction using a multimode microwave apparatus [28].Takeuchi et al. synthesized 60 nm Ti 4 O 7 nanoparticles via carbothermal reduction of TiO 2 nanoparticles using polyvinylpyrrolidone as the carbon source.The carbothermal reduction was carried out using 2.45 GHz microwave irradiation at 950 • C for 30 min.The results of this study demonstrate that microwave heating can drastically reduce the heating time to avoid excessive sintering and crystal growth of Ti 4 O 7 in a conventional carbothermal reduction process [65].Arif et al. prepared
[81].Chisaka et al. synthesized Ti 4 O 7 particles via carbothermal reduction, using titanium oxysulfate (TiOSO 4 ) and polyethylene glycol as precursors.The Pt catalyst using Ti 4 O 7 as support exhibited excellent load cycle durability, which was the highest among the state-of-the-art platinum/oxide catalysts, with no change in the cell performance after 10,000 voltage cycles [82].Esfahani et al. synthesized doped titanium sub-oxide Ti 3 O 5 Mo 0.2 Si 0.4 (TOMS) as a novel fuel cell catalyst support.Ti 3 O 5 Mo 0.2 Si 0.4 (TOMS) support exhibited remarkably high electronic conductivity and high stability.The fuel cell devices that used the Pt/TOMS catalyst achieved high performance, better than that of commercial catalysts [83].Nguyen et al. demonstrated the excellent durability of titanium sub-oxide as a catalyst support for Pd in alkaline direct ethanol fuel cells [84].Won et al. developed Ti 4 O 7 -supported Pt-based catalysts for a bifunctional oxygen catalyst in a unitized regenerative fuel cell for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) to enhance their activity and stability [85].Zhang et al. developed an ordered Ag@Pd alloy supported on Ti 4 O 7 .The ordered characteristics of the Ag@Pd alloy and its strong electron transfer with the corrosion-resistant Ti 4 O 7 improved the catalytic activity and stability [86].
5 ± 18.1 mmol h −1 m −2 at 793 L m −2 h −1 .The current efficiency was approximately 84%.These results show the high promise of REMs in applications of water treatment [94].Qi et al. prepared Ti 4 O 7 REM by thermal reduction of mechanically pressed TiO 2 powders, using the Ti powder as the reducing agent.The prepared Ti 4 O 7 REMs show high oxygen evolution potential and electrocatalytic activity for the generation of OH• [95].You et al. fabricated a monolithic porous Ti 4 O 7 electrode for electrochemical oxidation of industrial dyeing and finishing wastewater.The electrochemical oxidation using porous Ti 4 O 7 electrode produced efficient and stable reduction of recalcitrant organic pollutants onsite, without any extra addition of chemicals [96].Geng et al. fabricated tubular Ti 4 O 7 /Al 2 O 3 composite microfiltration membranes for electricallyassisted antifouling filtrations.The tubular Ti 4 O 7 /Al 2 O 3 membrane was tested for its antifouling performance by treating different feed solutions that are known to foul easily in an electrically-assisted membrane filtration module.The results demonstrated that the Ti 4 O 7 /Al 2 O 3 composite membranes showed much better antifouling performance than uncoated Al 2 O 3 membranes.The incorporation of a Ti 4 O 7 -modified membrane into the electrically-assisted filtration process provides a potential alternative for ceramic membrane filtrations to have antifouling properties for maintaining long-lasting permeate quality and simplifying the filtration operation [97].Liang et al. developed a REM system using a Ti 4 O 7 • C. The sequence of the formation of Magnéli series TiO x in the hydrogen reduction reactions of TiO 2 is Ti 9 O 17 , Ti 8 O 15 , Ti 7 O 13 , Ti 6 O 11 , Ti 5 O 9 and Ti 4 O 7 .

Table 1 .
Summary of synthesis of Ti 4 O 7 and other titanium sub-oxides.
• C were Ti 8 O 15 and unreacted TiO 2 , and the phase in a sample after being reduced to 975 • C was only Ti 4 O 7 .Ti 4 O 7 and Ti 3 O 5 phases were found at 1035 • C.

Table 2 .
A summary of methods for synthesis of nanostructured titanium sub-oxides.

Table 3 .
A comparative table among the synthesis methods.

Table 4 .
Electrical conductivity for single Magnéli phase materials *.

Table 5 .
Summary of application areas of Magnéli phases.